What do you mean by, "I changed it to MP3 and WAV files..."?  Do you mean you changed the file extension of the MMM arrangement file to MP3 or WAV?  If so, that will not work.  You need to leave the arrangement file as a .mmm file.

To create an audio file, you must EXPORT the arrangement as an audio file.  To do that, go to the "File" pull-down menu at the top left of the screen and click on EXPORT...

Then, you must choose the audio file format you wish to export.  You may also want to "normalize" and choose the bit rate for the export where appropriate.

I'd suggest spending some time familiarizing yourself with the program menus and interface.  Reading and understanding the owner's manual is always a good place to start.  You can access the owner's manual through the "Help" pull-down menu under Documentation.